When network interface cards were first developed, each card was assigned a unique MAC (Media Access Control) address that was “burned” into the memory of the interface card. Modern network interface cards support the ability to dynamically assign MAC addresses.
Many modern network interfaces support the dynamic assignment of MAC addresses. This ability has proven useful in high availability/fault tolerant applications, many of which use the ability to assign a MAC address in order to migrate a MAC address from one interface to another. One example of such usage is described in RFC 2338 from the Internet Engineering Task Force entitled “Virtual Router Redundancy Protocol” (VRRP). FTC 2338 defines a mechanism for providing virtual router IP addresses on a LAN to be used as the default first hop router by end-hosts. The advantage gained from using VRRP is a higher availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host While VRRP provides a virtual MAC address, it suffers from the problem that only 256 unique MAC addresses can be defined, which can be well short of the number required on many networks.